a/c blowing hot air on 1999 Chevrolet Silverado 1500

A/C is blowing hot. Have narrowed it down to temperature control. Compressor working. Lines are cold. Seems like the temperature control knob on the dash does not cause any difference in the air temperature coming out of the vents. What should I check now? Thank you!

The temperature door actuator behind the dash may have failed. If so a fault code should be stored in the a/c control module. Reading, diagnosing , and repairing the cause of any stored fault codes should get your a/c system back to proper operation. If you find yourself in the need of a repair shop please use the following link to find one near you - http://repairpal.com/directory?address=79065&car_brand_names=Chevrolet.

Thanks a bunch Oakman, I've been trying to resolve this problem for about 4 weeks now and I've replaced the resistor, a/c actuator and I was wondering if I had to replace the temperature control system. My e-mail is yukongreytiger@yahoo.com, so that I can relay the results back to you. Thank you again...

there a thing under the dash that open and closes a flap its behind a piece of plastic right where the passnger puts there feet it changes it from hot to cold or cold to hot look kinda squre has like threes screws in it that could be bad